Lithium ion technology is the leading technology in the field of rechargeable battery storage systems for portable electronics. Lithium ion batteries are being used as storage systems in mobile phones, camcorders and laptops, and have also been used for a while in battery-powered tools. The next step intended is the use of lithium ion batteries in larger systems, such as in automobiles or as stationary energy storage means for renewable energies. Because of the high cell voltage, their superior energy density and power density, and their notably low self-discharge, lithium ion batteries have a high potential for these applications. However, commercially available batteries still do not meet the elevated safety demands for large systems.
An important role is played here by the prevention of battery overcharging. In contrast to conventional batteries with aqueous electrolytes, such as the so-called lead accumulators, nickel-cadmium batteries or nickel-metal hydride batteries, lithium ion batteries having organic or polymeric electrolytes do not have any internal overcharge prevention. This prevention is guaranteed in the form of electronic control in the normal operation, for example, of an automobile by the battery management system (BMS). This monitors the voltage and temperature of each individual cell in the battery. In the event of failure of this system, however, overcharging of individual cells in the battery is possible and, as a result of this, overheating and explosion of the entire battery, since the rising temperature causes electrolyte components to become gaseous, and they can self-ignite. It is therefore necessary to integrate a further overcharge preventer into lithium ion batteries, which prevents the battery from overcharging in the event of failure of the external electronic control mechanisms.
An additional, internal overcharge preventer may be present in the form of a separator or of an additional layer within the battery, which melts when a particular temperature is exceeded and hence stops the current density flow. However, this offers prevention only from a critical point at which the overcharging process has already set in. Therefore, an overcharge preventer that prevents the overcharging process before there is a temperature rise, which is always of concern from a safety point of view, should be used. This can be achieved by means of electrolyte additives which, when there is a threat of overcharging, enter into an oxidation reaction and hence consume the charging current density. In this period, the charging current density is required for the reaction of the additive and the battery is not charged any further, as a result of which the voltage does not rise any further and hence the overcharging process is prevented.
These additives can be divided into two groups: the “redox shuttles” can be oxidized at the cathode and then reduced again at the anode to give the original additive, which means that they can be oxidized again at the cathode. However, this cycle is not implementable in practice, since the additive is consumed by other side reactions. A further great disadvantage is the low potentials at which many redox shuffle additives react.
A further group is that of the “non-redox shuttles”. This type of additives enters only into an oxidation reaction during the charging process, with consumption of the additive. The charging current density is required for decomposition and the battery is protected. The decomposition stops the battery operation and continuation of the cycling is not possible.
The best-known representative of this group is the molecule biphenyl, as disclosed, for example, in the document EP 0759641. A disadvantage of the use of biphenyl is, however, that the addition of biphenyl, even at a low concentration, has adverse effects on the battery properties. Furthermore, there are concerns that the presence of biphenyl in a fully charged cell, if it is stored over a prolonged period, will likewise exert an adverse effect on the battery properties. The potential of a fully charged cell is 4.3 V against Li/Li+. However, biphenyl has an oxidation shoulder of 4.3 V against Li/Li+. There is therefore a risk that there will be a slow but steady oxidation of biphenyl in the course of storage of a fully charged cell.